This invention relates to a servo control system responsive in general to a sequence of position commands supplied from an external source for intermittently and controllably driving a motor, with rotation of the motor fed back, to position a movable element at successively commanded positions substantially at rest. More particularly, this invention relates to a digital servo control system for use in a position control or positioning control system for either a carriage of a serial printer of the type known as an impact type in the art or a type wheel, drum, or cylinder carried by the carriage.
As will be described later with reference to a few of various figures of the accompanying drawing, an impact type serial printer is for use in combination with an electronic digital computer and comprises a carriage, a carriage motor for linearly moving the carriage along a predetermined path in either of two senses along the path, and a position control system for controllably driving the motor so as to successively and intermittently position the carriage at rest at positions commanded by the computer along the path. A rotary type wheel carrying a plurality of type elements is mounted on the carriage together with a type wheel motor for rotating the type wheel in either of two senses of rotation. A similar position control system controllably drives the type wheel motor so as to intermittently position the type wheel at rest at desired angular or rotational positions commanded also by the computer. Successively selected type elements are thereby placed at a common printing position predetermined relative to the carriage.
A position control system for controllably positioning a movable element, such as the carriage or the type wheel mentioned above, to a commanded position comprises an increment encoder or position transducer mechanically or otherwise coupled to a motor for the element. It is already known in the art to make the encoder produce a pair of incremental position signals having polarities that vary with a phase difference therebetween in response to every predetermined incremental angle of rotation of the motor. The successive variations in the polarity therefore correspond to a variation in the present or instantaneous angular position of the motor and hence the present linear or angular position of the movable element. The phase difference indicates one of a plus or positive and a minus or negative sense in which the present position is varying. The rate at which the polarities vary is proportional to a present angular velocity of the motor and accordingly the present linear or angular velocity of the element. The position signals are subtracted from the command signal to provide a position error between the present and the commanded positions. The motor is controlled so as to reduce the position error to zero.
In U.S. Pat. No. 3,954,163 issued to Andrew Gabor, a position control system for an impact serial printer is disclosed wherein either of a motor for a carriage and another motor for a rotary print wheel is controllably driven at first in a velocity control mode and then in a position control mode on moving the movable element intermittently from each commanded position to a next following commanded position. In the velocity control mode, the motor is driven to follow a reference angular velocity selected in consideration of the position error and represented by an analog reference velocity signal. This mode of operation is continued until the element is driven from a commanded position to a point spaced a predetermined length from the next following commanded position. In the position control mode, the motor is driven at a velocity related to an analog position error signal representative of the position error to eventually position the element at rest at the next following commanded position.
The use of analog signals in a servo control system used in such a position control system makes it difficult to achieve expectedly excellent results in operation and maintenance. Furthermore, it is rendered difficult to provide a compact servo control system. This means that the servo control system becomes bulky and expensive.
Shigeru Shimonou and Hiroshi Inada, assignors to the instant assignee, of whom Hiroshi Inada is one of the present joint inventors, have previously proposed a position control system in a copending patent application Ser. No. 911,263 filed May 31, 1978. The counterpart patent application in Germany was published in "Offenlegungsschrift" No. 28 23 571 on Mar. 1, 1979. It was believed that a servo control system used in the previously proposed position control system was much improved in these respects and, in fact, really is. The previous servo control system, however, is not operable completely on a digital basis. It is to be noted here that a digital servo control system suffers from a phenomenon called limit cycle. As will be discussed later with reference to a few of the various figures of the accompanying drawing, the limit cycle degrades performance of a digital servo control system.